UN High Commissioner for Human Rights Navi Pillay has opened a session of the Human Rights Council in Geneva, saying she is "gravely concerned" by the suffering of civilians in Syria.

Pillay said the Syrian government's use of heavy weaponry in civilian areas and extrajudicial killings and torture by antigovernment forces "may amount to war crimes and crimes against humanity," according to a UN statement on the session.

Pillay lamented that conditions to extend the mandate of the UN Observer Mission in Syria were not met and those operations ended on August 19.

Pillay reportedly expressed her full support for newly appointed Joint Special Representative Lakhdar Brahimi in his efforts to mediate talks in Syria.

Pillay also repeated calls for the international community to "overcome divisions and work to end violence and human rights violations" in Syria, the statement said.